In recent years, there has been a desire for a magnetic disk or sheet having excellent electromagnetic characteristics and which is suitable for high-density recording.
For the purpose of making magnetic disks suitable for high-density recording, ferromagnetic particles having small particle sizes are used, the surfaces of magnetic layers are made smooth or ferromagnetic particles are more uniformly dispersed in magnetic layers. Various methods are employed for these purposes. Further, in magnetic disks for high-density recording, the magnetic layers are becoming thinner. The production of magnetic disks for high-density recording by the above-described methods has caused problems in terms of the running durability of the thus-produced magnetic disks. That is, since such magnetic layers have smoother surfaces, the coefficients of the friction between the magnetic layers during running and the parts in recording and reproducing devices become so high that smooth running of the magnetic recording media is often inhibited or the magnetic layers are apt to suffer damage.
Furthermore, with the spread of VTR's, personal computers, word processors, etc., among the general public, the conditions, particularly, the temperature and humidity under which magnetic recording media are used, have a wide range of variability. Therefore, the above-described problem of running durability has become serious for rotary recording media, such as floppy disks for personal computers, word processors or the like, whose magnetic layers are subjected to repeated head-on-off operations, especially when such operations are conducted for a long period of time at high temperatures or under conditions of cyclically varying temperatures, from high temperatures to low temperatures.
In order to overcome the above problems, the following methods have been proposed: a method in which fatty acid esters are incorporated in the magnetic layers as disclosed in, for example, JP-A-50-22603, JP-A-50-153905, JP-A-55-139637, JP-B-39-28367, JP-B-41-18065 and JP-B-47-12950, a method in which a silicone compound is incorporated in the magnetic layers as disclosed in, for example, U.S. Pat. No. 2,654,681, and a method in which a fatty acid or a hydrocarbon is incorporated in the magnetic layers as discolored in, for example, U.S. Pat. Nos. 4,522,885, 4,711,673, 4,420,540, 3,634,253 and 4,548,873. (The terms "JP-A" and "JP-B" as used herein mean an "unexamined published Japanese patent application" and an "examined Japanese patent publication", respectively.)
However, the recording media produced by these prior art methods have a problem in that the above-described additives contained in the surface parts of the magnetic layers tend to vaporize at high temperatures, while the additives separate out on the surfaces of the magnetic layers at low temperatures, causing clogging of heads or frequent dropouts. Further, there is another problem in that in the case where the amount of the additive is increased to further improve the running durability, the additive undesirably plasticizes the binder resin and thus lowers the strength of the magnetic layer, whereby the durability becomes worse rather than improved.
In the case where ester compounds having a straight-chain alkyl group, which show relatively good lubricating properties, are used, they often cause the above-described problem that they separate out on the surfaces of the magnetic layers at low temperatures, because many of these have high melting points.
In attempts to overcome these problems, JP-B-47-12950, JP-A-58-160425, JP-A-58-218038, JP-A-60-205827, JP-A-61-294637 and JP-A-62-125529 disclose methods in which fatty acid esters having either a high-molecular-weight and branched hydrocarbon group or an unsaturated hydrocarbon group are incorporated in the magnetic layers, so that the fatty acid esters do not readily vaporize from the magnetic layers even at high temperatures and do not separate out on the magnetic layer surfaces at low temperatures. However, these esters cannot produce sufficient effects, since they plasticize the binder resins and thus lower the strength of the magnetic layers because they are liquid at room temperature and compatible with the binder resins. Moreover, the ester compounds having a branched alkyl group are also disadvantageous in that their lubricating properties are insufficient.